1. Field of the Invention
This invention is in the field of fluid transfer control and, particularly, in the area of providing safety during the transfer of flammable fluids, such as petroleum products.
2. Description of the Related Art
Controlling the safe and proper transfer of flammable fluids when loading transportation vehicles such as tanker trucks has long been a concern in the petroleum industry. In recent years, safety devices have been implemented on tanker trucks which prevent fluid transfer from a loading terminal to the truck if certain unsafe conditions surrounding the transfer exist. These devices use detection equipment to determine if all of the safety precautions have been taken, and inhibit fluid flow if they have not. The inhibiting of fluid flow is controlled electrically, by closing a valve in a fluid transfer conduit, or by disabling a pump which is responsible for transferring the fluid to the tanker.
FIG. 1 is a block diagram of a prior art system having control circuitry 10 which controls either the valve or pumping mechanism (or both) based on a number of different inputs. This figure demonstrates some of the input sources which are known in the art for controlling fluid transfer. Prior art systems may have some or all of the inputs shown in FIG. 1. If all of the necessary input signals are not in the proper state, the transfer of fluid is inhibited. In this manner, hazardous filling conditions are avoided.
Many fluid flow control systems use a real-time clock 12 such as that shown in FIG. 1. The clock input is used in conjunction with a memory unit of the control circuitry 10 to store time stamps indicative of when certain noteworthy events occur. That is, each time the system is operated to allow the transfer of fluid to or from a compartment of the tanker, the nature of the event is recorded in some encoded manner, along with the time as indicated by the input signal from clock 12. Thus, if any efforts are made to defeat the pump/valve control circuitry 10 (i.e. and transfer fluid under unsafe conditions) a record of the event is created. This acts as a deterrent to those who might try to engage in such a defeat of the system.
A "deadman" switch 14 has also been used which requires that an operator controlling the fluid transfer manually hold a switch mounted at the loading terminal closed during the entire loading or unloading process. This ensures that the operator is always present while the fluid transfer is taking place, so that an appropriate action may be taken if any problem occurs. The deadman switch 14 specifically addresses the problem of operators walking away from the equipment while a fluid transfer is underway.
ID sensor circuit 16 is typical of a truck identification system for which a memory unit is located on the truck in which is stored a unique identification (ID) number. When the truck is at the loading terminal, a signal line between the truck and the terminal is connected to allow the ID circuit 16 to access the memory unit on the truck to read the ID number. The truck ID number is then compared to a list of valid truck ID numbers, and the fluid transfer is inhibited if the truck's ID number does not match a number on the list. A system of this type is described in U.S. Pat. No. 5,534,856, which is assigned to the assignee of the present invention, and which is incorporated herein by reference.
The other input device shown in FIG. 1 is ground sensor circuit 18. One common safety concern during transfer of a flammable fluid is that of static electric discharges in the vicinity of the flammable fluid. A sufficient difference in the electrical potential of the tanker truck and a terminal from which it is loaded can result in an electrical arc which might ignite the nearby vapors of the fluid being transferred. For this reason, a commonly-accepted safety precaution is the establishment of a common electrical ground between the truck and the loading terminal. To ensure that such a common ground is established, non-defeatible ground sensor circuit 18 is used to verify the common ground, and inhibits fluid flow if the ground is not in place. An example of such a circuit may be found in U.S. Pat. No. 4,901,195, which is assigned to the assignee of the present invention, and which is incorporated herein by reference.
Another type of input is the overfill sensor circuit 13, of which a number of different types exist in the prior art. In general, the overfill sensor circuit consists of probes which detect when the fluid level in any of the compartments of a tanker truck exceeds a predetermined level. The control circuitry 10 responds to the indication of an overfill condition by discontinuing fluid flow to the truck.
While the various types of control inputs help ensure the safety of a fluid transfer operation, their effectiveness depends on the proper functioning of the control circuitry 10. Most such circuits tend to have switches which enable the pump or valve in question, but which are normally open when the system is off or when inputs to the control circuitry indicate that the fluid transfer should be disabled. However, if the control circuitry itself should malfunction in a manner which inhibits the ability to disable the fluid flow, an unsafe fluid transfer situation can result.